The present invention relates to acoustic ranging instruments including sonar systems, and more particularly to a system for improving coupled signals from a sonar transducer to a low-level pre-amplifier.
Transmit-Receive (T-R) switches are typically found in acoustic ranging systems including sonar systems. For long range sonar systems, relays are employed to couple a transmitter and receiver to a transducerxe2x80x94a common element to both the transmitter and the receiver. For example, a relay energizes to connect the transmitter to the transducer. After a high energy pulse is applied to the transducer, the relay de-energizes. During the receive cycle, another set of relay contacts connect the receiver""s input circuitry to the transducer. Typical relay switching times between transmit mode and the receive mode are on the order of 10-20 milliseconds, which corresponds to a minimum range of approximately 25-50 feet. The time interval between the end of a transmit pulse and the beginning of a receive signal is dead time having no usable information.
In order to eliminate the time-delay between relay switching times, short-range sonar systems use diode-resistor networks to provide near instantaneous switching between transmit and receive modes. The diode-resistor network has an advantage in that the receiver may remain coupled to the transducer. The receiver is protected from the high-voltage transmit pulse preferably by using steering diodes and resistors. Using the diode-resistor network, the time delay between a transmit mode and a receive mode is reduced to less than 50 microseconds, which corresponds to on or about a minimum range of less than 2 inches. The diode-resistor network, however, has a disadvantage in that the receiver portion of the network causes a loss of signal strength.
In one approach, T-R circuits provided by Matec Instruments delivered a transducer voltage transfer coupling ratio of 23.8% which is considerably lower than legacy T-R switch illustrated in FIG. 1. In another approach, T-R circuits provided by Western Marine Electronics delivered a coupling ratio of 14.9% which is lower than both the Matec design and the legacy T-R switch design as in FIG. 1. Western Marine""s circuit architecture uses a capacitive voltage divider (330 pF in lieu of R1 and 1803 pF in lieu of R2) but accomplishes a similar result. At 357 kHz, the transducer""s impedance is 247 ohms so that the coupling ratio is approximately (Vi*(247/(247+1351+246)))=Vi*0.134 (actual result may be closer to 0.149 when phase relationships are computed).
Therefore, it can be concluded that the above approaches thus couple about 14.9% to 23.8% of a received signal to the receiver amplifier. For a given low-noise receiver amplifier, this coupling loss decreases the signal-to-noise ratio by about 12.5 to 16.5 dB. If enough power were applied to the transducer, these losses would yield acceptable overall performance. However, the transfer ratios show that a significant amount of received signal has been lost. Once the low-level signals fall below the receiver amplifier""s noise floor, the chances of recovering the signal is small.
Thus, there is a need to achieve fast T-R switching times while ensuring that the received signal strength is not lost.
Accordingly, the present invention relates to an apparatus for achieving fast T-R switching times while ensuring that the signal strength of a received signal is not lost. A differential amplifier having a coupling resistor as an input is used so that the entire input signal is sensed by an operational amplifier, thus decreasing signal losses. Also, by increasing the value of the differential amplifier input resistors (R1-R3), signal loss is further minimized.
Thus, the present invention provides a novel transmit-receive (T-R) switch with fast T-R switching times and low signal loss overcoming the problems faced by prior approaches. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.
In one aspect, a transmit-receive (T-R) switching device for improving time-delay between relay switching times, the switching device comprising a differential receiver for achieving common mode rejection of undesired signals; and a coupling resistor for coupling a transducer to the differential receiver such that substantially all of an input signal is sensed by said receiver to reduce signal losses. The switching device further comprises a diode-resistor network for achieving fast transmit-receive switching times. Steering diodes and resistors are preferably used to protect the receiver from high voltage transmit pulses. The time-delay between a transmit mode and a receive mode is preferably less than 50 microseconds. The received signal strength is preferably reduced by the switching device to less than 1 dB. The differential receiver further comprises a differential amplifier; and wherein signal loss of the switching device is minimized by increasing resistance of the differential amplifier. The T-R switching device improves common mode interference rejection in a receive mode. The switching device is preferably used in sonar systems. For a given received signal, transmit signal is reduced to avoid risk of detection of sonar systems using the T-R switching device.
In another aspect, a method for improving time-delay between relay switching times of a transmit-receive (T-R) switch, the method comprising the steps of using a differential receiver for achieving common mode rejection of undesired signals; and coupling a transducer to the differential receiver using a coupling resistor such that substantially all of an input signal is sensed by said receiver to reduce signal losses. The method further comprising achieving fast transmit-receive switching times using a diode-resistor network, and providing steering diodes and resistors to protect said differential receiver from high voltage transmit pulses. The method also includes providing a differential amplifier; and minimizing signal loss of said switching device increasing the resistance of said differential amplifier.
In yet another aspect, a method for improving time-delay between relay switching times of a transmit-receive (T-R) switch, the method comprising: receiving and converting high-power alternating-current (AC) signals into high-level acoustic signals in a transmit mode; receiving and converting low-level acoustic signals into low-power electrical currents in a receive mode; steering low-power electrical currents to a differential receiver for sensing the low-power electrical currents; and coupling a transducer to the differential receiver using a coupling resistor such that substantially all of said low-power electrical currents are sensed by said receiver to reduce signal losses.
In another aspect, a method for improving time-delay between relay switching times of a transmit-receive (T-R) switch, the method comprising using a receiver having a differential amplifier for achieving common mode rejection of undesired signals; coupling a transducer to the differential receiver using a coupling resistor such that substantially all of an input signal is sensed by said receiver to reduce signal losses; and whereby transducer voltage transfer coupling ratio is provided by
Vi*(R1+R2+R3)/(Xi+R1+R2+R3) 
where Vi=transducer""s received input voltage
Xi=internal impedance of transducer
R1, R2=resistance of one leg of differential amplifier
R3=resistance of a second leg of differential amplifier
In a further aspect, a transmit-receive (T-R) switching device for improving coupled signals from a sonar transducer to a pre-amplifier, the device comprising a coupling resistor for limiting signals to the pre-amplifier during a transmit mode; the coupling resistor passing low-level signals to the pre-amplifier during a receive mode; and a differential amplifier using the coupling resistor such that substantially all of an input signal is sensed by the receiver to reduce signal losses.
In yet another aspect, an apparatus for improving time-delay between relay switching times of a transmit-receive (T-R) switch, comprising: means for receiving and converting high-power alternating-current (AC) signals into high-level acoustic signals in a transmit mode means for receiving and converting low-level acoustic signals into low-power electrical currents in a receive mode; means for steering low-power electrical currents to a differential receiver for sensing the low-power electrical currents; and means for coupling a transducer to the differential receiver using a coupling resistor such that substantially all of said low-power electrical currents are sensed by said receiver to reduce signal losses.
In a further aspect, an apparatus for switching signals between a transducer and a receiver in sonar systems, the apparatus comprising: a differential receiver for achieving common mode rejection of undesired signals, said receiver including a differential amplifier; a coupling resistor for coupling the transducer to the differential receiver such that substantially all of an input signal is sensed by said receiver to reduce signal losses; and whereby transducer voltage transfer coupling ratio is provided by
Vi*(R1+R2+R3)/(Xi+R1+R2+R3) 
where Vi=transducer""s received input voltage
Xi=internal impedance of transducer
R1, R2=resistance of one leg of differential amplifier
R3=resistance of a second leg of differential amplifier
In another further aspect, as shown in the exemplary inverting amplifier design of FIG. 3, a method for improving time-delay between relay switching times of a transmit-receive (T-R) switch, the method comprising: receiving and converting high-power alternating-current (AC) signals into high-level acoustic signals in a transmit mode receiving and converting high-level acoustic signals into low-power electrical currents in a receive mode; steering low-power electrical currents to an inverting receiver amplifier for sensing the low-power electrical currents; and whereby transducer voltage coupling ratio is unity as the transducer""s internal impedance forms an integral part of the inverting amplifier""s gain-setting network.